Shrink LittleFS partition, grow OTA app slots

Web assets only use ~489 KB but the default 1.375 MB spiffs
partition wasted the rest. Rebalance to 768 KB spiffs / 1.56 MB
per app slot (was 1.25 MB), giving the tight esp32 build real
headroom (99.6% -> 79.7% flash used) while preserving dual-slot
OTA. Wire the custom layout into both the local PlatformIO build
(new partitions.csv + platformio.ini) and the Arduino-CLI CI/release
pipeline (PartitionScheme=custom, matching littlefs image size and
merge_bin offset).

platformio.ini

@@ -0,0 +1,60 @@
+; PlatformIO configuration for ESPSomfy-RTS
+; Compatible with Arduino IDE (source files remain in project root)
+
+[platformio]
+src_dir = .
+default_envs = esp32
+
+[env]
+framework = arduino
+platform = espressif32@6.9.0
+monitor_speed = 115200
+upload_speed = 921600
+board_build.filesystem = littlefs
+board_build.flash_mode = qio
+board_build.partitions = partitions.csv
+lib_deps =
+    bblanchon/ArduinoJson@6.21.5
+    knolleary/PubSubClient@2.8.0
+    LSatan/SmartRC-CC1101-Driver-Lib@2.5.7
+    https://github.com/Links2004/arduinoWebSockets.git#2.4.0
+build_src_filter =
+    +<*.ino>
+    +<*.cpp>
+    +<*.h>
+    -<.github/>
+    -<icons/>
+
+[env:esp32]
+board = esp32dev
+
+[env:esp32c3]
+board = esp32-c3-devkitc-02
+board_build.mcu = esp32c3
+build_flags =
+    -DARDUINO_USB_MODE=1
+    -DARDUINO_USB_CDC_ON_BOOT=1
+
+[env:esp32s2]
+board = esp32-s2-saola-1
+board_build.mcu = esp32s2
+build_flags =
+    -DARDUINO_USB_CDC_ON_BOOT=1
+
+; ESP32-C6 requires Arduino ESP32 core 3.x (pioarduino platform fork).
+; Core 3.x has breaking API changes (ETH, tcpip_adapter removed, etc.)
+; that require source code modifications before this will compile.
+; [env:esp32c6]
+; platform = https://github.com/pioarduino/platform-espressif32/releases/download/53.03.11/platform-espressif32.zip
+; board = esp32-c6-devkitc-1
+; board_build.mcu = esp32c6
+; build_flags =
+;     -DARDUINO_USB_MODE=1
+;     -DARDUINO_USB_CDC_ON_BOOT=1
+
+[env:esp32s3]
+board = esp32-s3-devkitc-1
+board_build.mcu = esp32s3
+build_flags =
+    -DARDUINO_USB_MODE=1
+    -DARDUINO_USB_CDC_ON_BOOT=1